Principles of case control studies Part I • Nature of the design of case control studies • Odds ratio as the measurement of association Many slides in this presentation are from the World Health Organization and ization and Many slides in this presentation are from the World Health Organ the European Programme Programme for Intervention Epidemiology Training for Intervention Epidemiology Training, , thank you thank you. . the European Piyanit Tharmaphornpilas MD, MPH The I nternational Field Epidemiology Training Program, Thailand
Epidemiological studies 1. Descriptive 2. Analytical Intervention Clinical trials Observational Case control Cohort
Case control Study Exposure Disease (Case) ? ? No disease (Control)
Major Steps in case control study 1. Define and select cases ? ? Measles case: Children aged 0-10 yrs, living in BKK, who were diagnosed with measles by physicians during 2000-2005
Major Steps in case control study 1. Define and select cases Measles case: Children aged 0-10 yrs, living in BKK, who were diagnosed with measles by ? physicians during 2000-2005 ? 2.Select controls Control: Children aged 0-10 yrs, living in BKK, who were not diagnosed with measles by physicians during 2000-2005
Major Steps in case control study 1. Define and select cases 3. Ascertain exposures ? ? 2.Select controls Measles vaccination yes/no
Major Steps in case control study 1. Define and select cases 3. Ascertain exposures ? ? 2.Select controls 4. Compare exposure in cases and controls (Calculation of Odds Ratio)
Major Steps in case control study 1. Define and select cases 3. Ascertain exposures ? ? 2.Select controls 4. Compare exposure in cases and controls 5. Test any differences for statistical significance
Hypothesis in cohort study Is the incidence higher among exposed than non-exposed? Hypothesis in case control study Is the observed exposure among cases higher than expected? Thought If the frequency of exposure is higher among cases than controls, then the incidence will probably be higher among exposed than non-exposed
Example Is the proportion of oral contraceptive use among myocardial infarction cases higher than among controls? (Is the observed exposure among cases higher than expected?) If the proportion of oral contraceptive use is higher among MI cases than controls, then the MI incidence will probably be higher among oral contraceptive users than among non-users
Distribution of myocardial infarction cases and controls by oral contraceptive use Oral Myocardial contraceptives Infarction Control Yes 693 320 No 307 680 Total 1000 1000 % exposed 69.3% 32 %
Cohort studies � Risk � Risk difference � Relative risk (strength of association) Case control studies � No calculation of rates, so no relative risk � Proportion of exposure Any way of estimating Risk & Relative risk ???
Probability that an event will happen Odds = Probability that the even will not happen Odds Probability that an event will happen Odds = 1 - (Probability that the event will happen) Foot ball game Wins Loose Total Team A 2884 97116 100000 Risk of winning = 2884 / 100000 = 0.029 Risk of not wining = 1 - 0.029 = 0.971 Odds of wining = 2887 / 97116 = 0.029/0.971 = 0.030
Odds ratio in case control study Odds of exposure among cases = Cases Controls Probability to be exposed among cases Probability to be unexposed among cases Exposed a b a / (a+c) Not exposed c d Odds Ec = ------------ = a / c c / (a+c) Total a + c b + d Odds of exposure among controls = Probability to be exposed among controls a/c Probability to be unexposed among controls OR = ---- = ad / bc b/d b/ (b+d) Odds Ec = ------------ = b / d d/ (b+d)
OR in case control is a good estimate of RR ( if disease is rare )
Distribution of myocardial infarction cases and controls by recent oral contraceptive use Oral Myocardial contraceptives Infarction Control OR Yes 693 320 4.8 No 307 680 Ref. Total 1000 1000 693 * 680 Oral contraceptive users are 4.8 times more likely than non users to OR = = 4.8 have myocardial infarction 320 * 307
End of part I
Recommend
More recommend
